Pigments are important components in paint and coating compositions as they provide the requisite degree of opacity and affect other coating properties such as TiO.sub.2 augmentation, sheen, gloss, scrubability, stain removal and tinting strength. The level of opacity, or hiding power, is a critical performance property of most coatings. Opacity can be generally defined as the degree to which a material obscures a substrate. Some key factors affecting opacity are: (1) the difference in the refractive index between the pigment and the medium; (2) the spacing of the pigment particles; (3) the size of the pigment particles, and (4) the introduction of air microvoids in the coating through the use of structured pigments. To achieve opacity, there must be a significant difference in the refractive index between the pigment and the medium used.
The most effective opacifying, white pigment used today is titanium dioxide (TiO.sub.2). TiO.sub.2 yields the greatest difference in the refractive index between any pigment and commonly used coating mediums. TiO.sub.2 extension has been a desired goal in the coatings industry for some time for the several reasons: (1) TiO.sub.2 is the largest line item in a coating producer's raw material budget; (2) coating producers are constantly looking for ways to reduce costs; (3) TiO.sub.2 has a relatively high unit price, and (4) there exists a possibility of a shortage of TiO.sub.2.
It is generally known that TiO.sub.2 extension can be achieved through the use of: ultra-fine particle size pigments, preferably having a particle size in the range of about 0.2.mu. to about 0.3.mu.; structured pigments which contain air voids, and a combination of ultra-fine particle size pigments and structured pigments as previously described in U.S. Pat. No. 5,112,402. The first strategy involves spacing the individual titanium dioxide particles in order to obtain optimal use of the remaining TiO.sub.2 which is added to the coating. It has been determined that for optimum efficiency of TiO.sub.2, there should be a 0.2.mu. to a 0.3.mu. space between the TiO.sub.2 particles. The second strategy utilizes high oil absorption, structured pigments to introduce air voids in the dried coating film. The air voids, like TiO.sub.2, produce boundary areas of differing refractive indexes which refract light and thereby provide additional dry hiding. The pigments which promote titanium dioxide extension are known as TiO.sub.2 extenders.
Structured pigments used as titanium dioxide extenders are typically available only in dry form. Such extenders include flash calcined clays (e.g. OPTIWHITE calcined clay available from Burgess Pigment of Sandersville, Ga.), standard calcined clays (e.g. HUBER 70C calcined clay available from J. M. Huber Corp., Edison, N.J.), and amorphous silicates (e.g. ZEOLEX 80 Amorphous Silicate available from J. M. Huber Corp.). Utilizing these dry form extenders in a paint (or other coating) formulation presents several problems. Coating manufacturers, who purchase the dry form extenders, must make a dispersion of the pigments (i.e. extenders) at their respective plants. A substantial amount of dust is created when the bags, which contain the pigments, are opened in order to be made into a dispersion. Further, the storage of the bags of pigments require significant amounts of space. Other drawbacks include poor batch to batch consistency and increased machine wear resulting from the grinding and milling steps required to make the paint.
In recognition of the foregoing, a unique slurry system was developed and is disclosed in U.S. Pat. No. 5,112,402. From the coatings manufacturer's point of view, there are several advantages for purchasing a pigment slurry as opposed to purchasing the pigments in dry form. Some of the advantages include: reduction of dust and bag disposal, reduction of storage space requirements, improved production speed (since no grinding or milling is required), minimization of dispersion, reduction of machine wear and repair costs, improvements in consistency and coating properties, and the ability to readily make corrections to finished formulations.
However, the slurry disclosed in the '402 patent does not have the sheen control required by certain coating compositions such as flat latex paint formulations. Further, such slurry has a relatively high binder demand and does not have the requisite solids percentage for certain coating formulations.